Practice portal › Wave Optics › Diffraction
Asked in GSEB Board March 2022 · Single slit diffraction
Given: a slit of width a with a lens of focal length f right behind it, light of wavelength λ at normal incidence, and the pattern collected in the focal plane of the lens.
Idea: the lens brings all the light diffracted in one direction θ to a single point at y=fθ in its focal plane, so f plays the part the screen distance plays without a lens.
The first minima of a single slit lie at sin θ=±λ/a, and the angle is small, so θ≈λ/a.
Half-width on the screen: y=fθ=(fλ)/a.
The central maximum reaches the first minimum on each side of the axis, so its size is 2y.
Size =(2fλ)/a.
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